Berkeley Lab researchers
have produced non-toxic magnesium oxide nanocrystals that efficiently emit blue
light and could also play a role in long-term storage of carbon dioxide, a potential
means of tempering the effects of global warming.
This is a high resolution electron micrograph image of magnesium oxide nanocrystals; the inset shows a single nanocrystal. Credit: Lawrence Berkeley National Laboratory
In its bulk form, magnesium oxide is a cheap, white mineral used in applications
ranging from insulating cables and crucibles to preventing sweaty-palmed rock
climbers from losing their grip. Using an organometallic chemical synthesis
route, scientists at the Molecular Foundry have created nanocrystals of magnesium
oxide whose size can be adjusted within just a few nanometers. And unlike their
bulk counterpart, the nanocrystals glow blue when exposed to ultraviolet light.
Current routes for generating these alkaline earth metal oxide nanocrystals
require processing at high temperatures, which causes uncontrolled growth or
fusing of particles to one another-not a desirable outcome when the properties
you seek are size-dependent. On the other hand, vapor phase techniques, which
provide size precision, are time and cost intensive, and leave the nanocrystals
attached to a substrate.
"We've discovered a fundamentally new, unconventional mechanism for nicely
controlling the size of these nanocrystals, and realized we had an intriguing
and surprising candidate for optical applications," said Delia Milliron,
Facility Director of the Inorganic Nanostructures Facility at Berkeley Lab's
nanoscience research center, the Molecular Foundry. "This efficient, bright
blue luminescence could be an inexpensive, attractive alternative in applications
such as bio-imaging or solid-state lighting."
Unlike conventional incandescent or fluorescent bulbs, solid-state lighting
makes use of light-emitting semiconductor materials-in general, red, green and
blue emitting materials are combined to create white light. However, efficient
blue light emitters are difficult to produce, suggesting these magnesium oxide
nanocrystals could be a bright candidate for lighting that consumes less energy
and has a longer lifespan.
These minute materials do more than glow, however. Along with their promising
optical behavior, these magnesium oxide nanocrystals will be a subject of study
in an entirely different field of research: Berkeley Labs' Energy Frontier Research
Center (EFRC) for Nanoscale Control of Geologic CO2, designed to "establish
the scientific foundations for the geological storage of carbon dioxide."
Experts say carbon dioxide capture and storage will be vital to achieving significant
cuts in greenhouse gas emissions, but the success of this technology hinges
on sealing geochemical reservoirs deep below the earth's surface without allowing
gases or fluids to escape. If properly stored, the captured carbon dioxide pumped
underground forms carbonate minerals with the surrounding rock by reacting with
nanoparticles of magnesium oxide and other mineral oxides.
"These nanocrystals will serve as a test system for modeling the kinetics
of dissolution and mineralization in a simulated fluid-rock reservoir, allowing
us to probe a key pathway in carbon dioxide sequestration," said Jeff Urban,
a staff scientist in the Inorganic Nanostructures Facility at the Molecular
Foundry who is also a member of the EFRC research team. "The geological
minerals that fix magnesium into a stable carbonate are compositionally complex,
but our nanocrystals will provide a simple model to mimic this intricate process."
Hoi Ri Moon, a post-doctoral researcher at the Foundry working with Milliron
and Urban, noted her team's direct synthesis method could also be helpful for
already-established purposes. "As a user facility that provides support
to nanoscience researchers around the world, we would like to pursue studies
with other scientists who could use our nanocrystals as 'feedstock' for catalysis,
another application for which magnesium oxide thin films are commonly used,"
said Moon.
More information: "Size-controlled synthesis and optical properties of
monodisperse colloidal magnesium oxide nanocrystals," by Hoi Ri Moon, Jeffrey
J. Urban and Delia J. Milliron, appears in Angewandte Chemie International Edition
.